That happened to me once. I had self injured and I didn't lose enough blood for that to be the cause, but I got really lightheaded and even fell twice. I still couldn't tell you why, but I almost passed out.
It sounds like psychological shock to me. For me, it only happened once, it never occurred again, but if it does, I would recommend sitting or lying down, in case you stumble or fall. You could hurt yourself really bad.
Sorry if this comment wasn't much help, I hope you stay safe. Hugs :)
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